Respect Dropbox TOS and your files will not be deleted.
Share some copyrighted material and deal with the consequences.
By the way you can encrypt files before sending them to the cloud, but in order to share them with a third party you'd have to give your keys, which makes little sense.
Example, my wife and I are physically separated (let's say she is in Europe for a week) and she wants a copy of "Frozen" that I just ripped from the DVD I own. (It's a long plane ride home.)
Making a copy is fair use, and putting a copy on her computer is not illegal.
Dropbox is treading a fine line. If they fuck with my files, I will drop them fast.
https://tahoe-lafs.org/trac/tahoe-lafs
(One of the properties of Tahoe is managing the keys to decrypt data completely outside of the servers, so that the servers never have any knowledge of what they're storing.)
Don't do illegal shit on Dropbox; problem solved.